In the practice of the present invention the crowns of the work rolls in a continuous roll casting machine are controlled by providing variable cooling internally to the rolls.
It is known to use water to internally cool work rolls as disclosed in U.S. Pat. Nos. 3,757,847 to Sofinsky et al., and 4,671,340 to Larrecq et al. Effective cooling is not only necessary for prolonging the life of work rolls, but is also necessary to withdraw heat from the metal being roll cast.
Controlling the temperature of work rolls is also desirable for maintaining a constant distance between rolls during the roll casting operation. If the temperature of a work roll is permitted to increase, its perimeter will move outward due to its thermal expansion, reducing the thickness of the sheet being roll cast.
As well as controlling the overall temperature of work rolls, it is also desirable to control the temperature in various portions of a roll. The center of a work roll tends to heat up more than its ends, resulting in the formation of a thermally induced crown on the roll. As little as a ten degree differential between the center and the ends of a roll may cause a crown to develop.
A limited amount of crowning is desirable to offset the bending of the work rolls by the sheet being cast. However, excessive crowning will cause sheet to be roll cast thinner in its center portion than at its edges. This is undesirable when the sheet is to be cast flat, for example, when foil will be made from the sheet. It is also undesirable for most other products where sheet is preferably roll cast slightly thicker, rather than thinner in its center, to allow the sheet to be self centering during subsequent rolling operations. Control of the crown of work rolls is therefore desirable to permit control of the shape of the sheet being roll cast.
Current internal work roll cooling systems may provide greater cooling to the center of the roll than to its ends to control excessive crowning. However, the relationship between the amount of cooling water circulating in the center of the roll and its ends is fixed. Due to the variability of cooling requirements caused by the roll casting of different metals at differing thicknesses, excessive work roll crowning may still occur with these internal cooling systems.
Water may be sprayed on the exterior of work rolls in a rolling mill to provide differential cooling as disclosed in U.S. Pat. No. 3,784,153 to Ross et al. External cooling of work rolls, however, is practical only for machines having rolls of a relatively small diameter, such as the type used for finishing work. Larger work rolls, have too great a mass and heat input from the molten metal to be responsive to water sprayed on their perimeters.
External cooling of work rolls in a casting machine, additionally, has notable disadvantages. If a significant amount of cooling water should contact the molten metal being cast, the rapid expansion of the water into steam may cause molten metal to be sprayed out from the casting machine, causing a danger to nearby personnel. External cooling water may also be damaging to equipment. The carriers, guides and feed tips which provide molten metal to roll casting machines are made with asbestos or ceramic materials which are easily damaged by exposure to water.
Thus, there exists a substantial need for an improved system to better control the crown of work rolls in roll casting machines, and the crown of sheet produced by such machines, without the drawbacks of the systems discussed above.